Spread spectrum communication is designed to spread data having a relatively narrow bandwidth into a relatively wide frequency band by modulating it as a spread code sequence. Thus, this communication method requires in essence relatively low transmission power per unit frequency, and it leads to minimization of interference to other communication operations and excellent withstandability against environmental noises incoming during the transmission process.
FIG. 6 is a block diagram showing the general construction of a system for performing spread spectrum communication through radio communication. In the same drawing, transmitter TX.sub.1 performs product modulation of a spread code sequence generated from data D through sequence generator 1, and modulates the carrier wave having frequency f.sub.0 generated by oscillator 2 using the product-modulated signal. The carrier wave including data D subjected to such a spread spectrum process is then transmitted to receiver RX.sub.1 through radio communication. In that case, a pseudo noise sequence (PN sequence) having the same bit periodic length as that length of data D is generally used as the spread code sequence. Thus, the related art is described here with reference to the so-called M-sequence which is used most widely among these PN sequences.
Transmitter RX.sub.1 introduces the spread-spectrum-modulated signal into amplifier 3 through an antenna (not shown) to amplifier it to a desired level. Then, the signal processed by frequency mixing with local signal f.sub.L (.apprxeq.f.sub.0) of local oscillator 4 is demodulated into a spread signal of a base band through low pass filter 5. Thereafter, the base band spread signal and an M-sequence code identical to that used in the transmitter TX.sub.1 and generated from sequence generator 6 are supplied to multiplier 7. The output of multiplier 7 is and then integrated for a period of one frame of the M-sequence by integrator 8 that produces the cross-correlation of these signals. Further, the so-obtained signal is demodulated into the original data D by detector 9 at the end point of the frame. Then, the demodulated data is supplied to a control terminal of the sequence generator 6 through chip (and/or frame) timing synchronization detector 10 to synchronize the M-sequence generation timing with the phase of the received signal.
FIG. 7 is a diagram schematically showing spectra of signals in the transmission process, wherein reference numeral 11 designates a spectrum of the spread-spectrum-modulated signal while 12 designates that of the incoming environmental noise. Namely, by the demodulation (inversion) using the M-sequence in the receiver, as shown in FIG. 8, the spread-spectrum-modulated signal 11 having been spread in a relatively wide frequency band is transformed into signal 13 with a relatively narrow bandwidth while the environmental noise 12 is transformed into signal 14, spread in a relatively wide frequency band. Therefore, this method is now considered as a communication method capable of minimizing the influence of environmental noises.
However, the spread-spectrum-modulated signal in such a system is spread over an extremely wide frequency range. Thus, in the event that the signal reaches the receiver through a transmission system such that colored noise like the environmental noise 12 of FIG. 7 having highly concentrated power in a relatively narrow band present at plural points over that frequency range, the noise level 14 after the inversion due to the M-sequence is drastically increased despite the spreading effect given by the M-sequence, thereby degrading the S/N ratio.
The present invention was made to solve the problem of the conventional spread spectrum communication system as mentioned above. Therefore, it is an object of this invention to provide a signal receiving system for spread spectrum communication which is improved in the receiving S/N characteristics by reducing the influence of colored noise having a relatively narrow band spectrum which is added to the receiving signal through the transmission process.